This is a general program in the diagnosis and recognition of behavioral patterns in the tumors of laboratory animals employing techniques in light and electron microscopy and histochemistry. Major interest is currently focused on tumors of the Osborne-Mendel rat, since more than 4,000 aged females of this strain have been necropsied in this laboratory. This project has been phased out except for 9 autopsies on aged OM female rats (149-159 weeks old).